A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In addition to EUV radiation, an EUV source emits many different wavelengths of radiation and debris. This non-EUV radiation is harmful for an EUV lithography system, so it has to be removed by, for example, a spectral purity filter. Present spectral purity filters are based on blazed gratings. However, these gratings are difficult to produce, since the surface quality of a triangular shaped pattern on the spectral purity filters has to be very high. The roughness of the surface should be lower than 1 nm RMS.
Debris mitigation schemes may be applied for suppressing debris emitted from radiation sources. However, commonly used debris mitigation methods, which include foil traps and gas buffers, do not guarantee effective debris protection. Moreover, use of standard (e.g. Zr) thin filters transmissive for EUV is virtually impossible due to the fragility of the filters and low heat-load threshold.
Debris mitigation schemes may also involve physical removal of components from a lithographic apparatus and their off-line cleaning using chemical processes. However, having to accommodate such off-line cleaning makes vacuum and mechanical design of a lithographic apparatus extremely complicated. Off-line cleaning also involves a significant amount of down time for the lithographic apparatus.
A further problem of existing spectral purity filters is that they change the direction of a radiation beam from an EUV source. Therefore, if a spectral purity filter is removed from an EUV lithography apparatus, a replacement spectral purity filter has to be added or a mirror at a required angle has to introduced. The added mirror introduces unwanted losses into the system.